Extracellular (ECF) K+ homeostasis is critical for normal cardiovascular and neuromuscular functions and is maintained by renal and extrarenal mechanisms. We recently developed the K clamp technique, which can quantify both renal K+ excretion and extrarenal cellular K+ uptake in vivo. Using this technique, we demonstrated that both renal K+ excretion and extrarenal cellular K+ uptake are rapidly and profoundly suppressed during K+ deprivation. These changes for ECF K+ conservation have been traditionally explained to arise from decreased ECF K+ levels and/or consequent decrease in aldosterone secretion. However, we found that this triggering of ECF K+ conservation can occur in rats in the absence of changes in plasma [K +] or [aldosterone] when K+ intake is reduced to 1/3 of control. The objective of this project is to test the hypothesis that K+ intake is sensed by K+ sensors in the portal vein, and both renal K+ excretion and extrarenal cellular K+ uptake are regulated by this signal. To achieve this goal, we will collaborate with Dr. Casey Donovan at our institution and employ strategies similar to those used to demonstrate the presence of portal vein glucose sensors in rats, i.e., "local irrigation" and portal denervation techniques. Our preliminary data showed that reducing K+ intake to 1/3 of normal for only one night (12 h) was sufficient to trigger marked renal K+ conservation. In the proposed studies we will use this overnight low K+ feeding model to address the following specific aims. Aim 1. Test for the existence of portal (or splanchnic) sensing of Kv intake and its regulation of renal and extrarenal K+ handling. We will test whether the triggering of ECF K+ conservation by overnight low dietary K+ intake is prevented by parallel K+ supplementation via intragastric or intraportal infusion, which would be sensed by the hypothetical portal vein (or splanchnic) sensors, but not by K+ supplementation via systemic infusion. Aim 2. Test for a role of portal sensing of K+ intake in extracellular K+ homeostasis. We propose to test whether portal denervation ablates portal sensing of Kv intake and, if so, whether it impairs the acute and chronic regulation of ECF K+ homeostasis by delaying renal and extrarenal responses to altered K+ intake. Significance: This project will potentially identify an important, previously unknown regulator of ECF K+ homeostasis.